1. Field of the Invention:
This invention relates to a light measuring device for a camera and particularly to a light measuring device which is capable of selectively using some of a plurality of light sensitive elements assigned to different light measuring areas.
2. Description of the Prior Art:
There have been known light measuring devices for cameras of the kind arranged to use a plurality of light sensitive elements and to be shiftable between an averaging light measuring mode and a spot light measuring mode by selecting some of these light sensitive elements.
The method employed in the light measuring devices of the prior art for shifting one light measuring mode to the other is as shown in FIGS. 1 and 2 of the accompanying drawings. For simplification of illustration, let us assume that two light sensitive elements are used in the light measuring device and that either one of these light sensitive elements is selected by means of a change-over switch to accomplish a light measuring operation selectively in the averaging light measuring mode or in the spot light measuring mode.
FIG. 1 is a plan view of an example of the prior art light sensitive element arranged to be used in the light measuring device. Two light sensitive elements such as SPC (silicon photo-cells) or the like are formed into one unified light receiving element 1. A first light sensitive element 1a which is for the spot light measuring mode is arranged to measure the brightness of a part in the middle portion of a picture plane to be used in photographing with the light receiving element 1 set in a known light receiving position (not shown) of the camera. Another light sensitive element 1b which is for the averaging light measuring mode is arranged to measure the brightness of the remaining portion of the picture plane excluding the middle part to be measured by the light sensitive element 1a.
As shown in FIG. 2, these light sensitive elements 1a and 1b are connected in parallel with each other via a change-over switch 5. In the drawing, the change-over switch 5 is shown as a mechanical switch for the sake of simplification of illustration. Actually, however, this switch is arranged to be a semi-conductor switch. The output terminals of these light sensitive elements are connected respectively to the two input terminals of an operational amplifier 3 (hereinafter an operational amplifier will be called OP amp. for short). The OP amp. 3 is of a high input impedance and uses a MOSFET (an MOS type field effect transistor) or the like for the first stage of its output terminal. To the feedback route of this OP amp. 3 is connected a diode 4 which logarithmically compresses the photo-currents of the above-stated light sensitive elements. With the prior art light measuring device arranged in the manner as described above, when the switch 5 is closed, the sum of the photocurrents of the light sensitive elements 1a and 1b is logarithmically compressed by the diode 4 and appears at the output terminal 3A of the OP amp. 3 as a light measurement value in the form of a voltage. In other words, averaging light measurement is carried out in this manner.
Further, when the switch 5 is opened, the photo-current produced from the light sensitive element 1b does not flow to the diode 4. Meanwhile, the photo-current from the other light sensitive element 1a alone is logarithmically compressed and converted into a voltage. This voltage is then produced as a light measurement value from the output terminal 3A of the OP amp. 3 and is applied to an exposure control circuit such as an electric shutter arrangement. Thus, a spot light measuring operation is carried out.
In the case of the light measuring device of the prior art, the above-stated spot light measurement can be carried out when the change-over switch 5 is completely opened. Actually, however, there exists a resistance called "off resistance," which makes a completely open state of the switch not readily attainable. Therefore, the light measuring device of the prior art has been incapable of carrying out accurate spot light measurement. In other words, in the case of the prior art device, a portion of the photo-current produced from the light sensitive element 1b flows via the above-stated "off resistance" to the diode 4 during a spot light measuring operation and this hinders correct spot light measurement which should be accomplished solely on the basis of the photo-current of the light sensitive element 1a. Further, this light measurement error resulting from the "off resistance" increases in proportion to the ratio of the photo-current of the light sensitive element 1a to that of the light sensitive element 1b produced when the switch 5 is closed. More specifically, the error increases with area ratio of the light receiving surface of the light sensitive element 1a to that of the light sensitive element 1b (assuming that the light sensitive element 1 has uniform sensitivity irrespectively of difference in position thereof). Therefore, the narrower the spot region of the spot light measurement (the light receiving area on the surface of the light sensitive element 1a), the more the light measuring error becomes unallowable.
Another shortcoming of the prior art light measuring device has resided in that: When the switch 5 is in an open condiiton, the light sensitive element 1b which has one terminal thereof in an open state comes to be connected to the inversion input terminal of the OP amp. 3. As a result when the quantity of light to be received by the light sensitive element 1b is small and the impedance of the light sensitive element 1b becomes large in particular, the device becomes vulnerable to an induced noise, which is apt to result in a light measurement error.